Piezoelectric fuel injectors are well-known for use in automotive engines and employ a piezoelectric actuator, made of a stack of piezoelectric elements arranged mechanically in series, for opening and closing an injection valve to meter fuel injected into the engine. One type of piezoelectric fuel injector is the de-energize-to-inject injector described in EP174615. The injector stack is held in a charged state during periods of non-injection, and when it is required to inject fuel the stack is de-energized. When injection is to be terminated the stack is re-charged again. In an energize-to-inject injector, operation is reversed so that charging of the stack initiates injection and discharging of the stack terminates injection.
Piezoelectric actuators, and hence fuel delivery, are controlled by an engine control module (ECM). The ECM incorporates strategies that determine the required fuelling and timing of injection pulses based on the current engine operating conditions, including torque, engine speed and operating temperature. Such strategies determine the number, size and timings of the injections and tend to be large and complicated. Furthermore, such strategies are calibrated for specific applications (i.e., specific customers and specific engines).
Strategies of this type allow for multiple injection pulses, such as pilot and post injections. Pilot injections are generally used to reduce combustion noise, and make the engine sound less like older diesel engines. Post injections are generally used in a couple of ways: close to the main injection they are used to reduce soot (this is sometimes referred to as split main); and late post injections are used for aftertreatment systems, i.e., deNOx filters and particulate traps.
Although pilot injections are used in diesel engines to reduce combustion noise, they can lead to an increase in smoke production. Minimising the separation between the pilot and main pulses can improve the smoke-noise tradeoff, i.e., achieving good noise reduction with smaller increases in smoke.
The quantity, fuelling and timing of these injection pulses is continuously variable across the engine operating range. This allows optimization of the engine operation in terms of performance, fuel economy and emissions.
The ECM selects the injector to be opened and determines when the injector is to be opened, how long it is to remain open before being closed (this is known as an injection event), and for how long the injector is to remain closed before the next injection event.
The time separation between one injection event and another, i.e., the time period between a termination (i.e., conclusion) of an electrical on signal associated with the first injection event and an initiation of an electrical on signal associated with the second injection event, is known as the demand time, and is controlled by the ECM depending on the current operating strategy (i.e., driver demands and current engine operating conditions).
Being able to control the demand time accurately is key to the flexibility of the ECM. It allows optimization in terms of engine performance, noise and other unwanted emissions, for example nitrous oxides and particulates.
In known injectors of the de-energize to inject type, the stack is charged fully to ensure that the electrical charge across the stack returns to a known level, providing a reference for the next discharge phase. As a result, there is a limit to how short the demand time can be because it is governed by the time required to charge the stack fully, the time it takes to open the injector, and the time required for the switching means controlling the injection to switch on and off as appropriate. However, in order to increase flexibility of operation it is desirable to reduce the demand time beyond the limit imposed by known injection control strategies.